0D-1D codes allow researchers to obtain a prediction of the behavior of internal combustion engines with little computational effort. One of the submodels of such codes is devoted to the centrifugal compressor. This model is often based on the compressor performance maps, therefore requiring the extrapolation of the maps so that all possible operating conditions are covered. Particularly, a suitable extrapolation of isentropic efficiency map is sought. This work first examines different available methods for compressor efficiency extrapolation into off-design conditions. No method is found to provide satisfactory results at all extrapolated regions: low and high compressor speeds and low compression ratio at measured speeds. Hence, a new method is proposed and its accuracy is assessed with the aid of compressor off-design measurements. The developed procedure is based on the normalization of the efficiency iso-speed lines, which are mathematically fitted to a constrained two-parameter function. These parameters are then extrapolated taking into account compressor theoretical considerations. Maximum efficiency and corrected mass flow at unity compression ratio are the parameters used to normalize the efficiency iso-speed lines. Its evolution against compressor speed is also fitted so that these variables can be extrapolated at other compressor speeds. In this way, the normalization can be undone to obtain the actual efficiency iso-speed lines. The proposed method presents higher accuracy than the reviewed ones in terms of efficiency extrapolation. Moreover, smooth transitions are considered between the actual efficiency values in the measured range and the extrapolated values at low compression ratio.